Conductive polymers, or more precisely intrinsically conducting polymers (ICPs), are organic polymers that conduct electricity. Such compounds may have metallic conductivity or be semiconductors. The biggest advantage of conductive polymers is their processability. As conductive polymers are also plastics, they can combine the mechanical properties (flexibility, toughness, malleability, etc.) of plastics with high electrical conductivity. Moreover, the properties of these materials can be fine-tuned using conventional organic synthesis techniques. (See, e.g., György Inzelt, Conducting Polymers A New Era in Electrochemistry. Springer. pp. 265-269 (2008), the disclosure of which is incorporated herein by reference.)
In traditional polymers, such as polyethylenes, the valence electrons are bound in spa hybridized covalent bonds. Such “sigma-bonding electrons” have low mobility and do not contribute to the electrical conductivity of the material. The situation is completely different in conjugated polymer materials. Conducting or conjugated polymers have backbones of contiguous sp2 hybridized carbon centers. One valence electron on each center resides in a pz orbital, which is orthogonal to the other three sigma-bonds. The electrons in these delocalized orbitals have high mobility when the material is “doped” by oxidation, which removes some of these delocalized electrons. Thus these conjugated polymer systems form a one-dimensional electron band structure, and the electrons within this band become mobile when it is partially emptied. In addition, these same materials can be doped by reduction, which adds electrons to an otherwise unfilled band. Although typically “doping” conductive polymers involves oxidizing or reducing the material conductive organic polymers associated with a protic solvent may also be “self-doped.”
The most notable difference between conductive polymers and inorganic semiconductors is the electron mobility, which until very recently was dramatically lower in conductive polymers than their inorganic counterparts. The fundamental low charge carrier mobility is related to the inherent structural disorder of these materials. In fact, as with inorganic amorphous semiconductors, conduction in such relatively disordered materials is mostly a function of mobility gaps with phonon-assisted hopping, polaron-assisted tunneling, etc., between localized states. And, more recently, it has been reported that quantum decoherence on localized electron states might be the fundamental mechanism behind electron transport in conductive polymers. (See, e.g., McGinness, John E., Science 177 (52): 896-897 (1972), the disclosure of which is incorporated herein by reference.)
Conjugated polymers in their undoped, pristine state are semiconductors or insulators. As such, the energy gap can be >2 eV, which is too great for thermally activated conduction. Therefore, undoped conjugated polymers, such as polythiophenes and polyacetylene only have a low electrical conductivity of around 10−10 to 10−8 S/cm. However, even at a very low level of doping (<1%), electrical conductivity increases several orders of magnitude up to values of around 0.1 S/cm. Subsequent doping of the conducting polymers typically results in a saturation of the conductivity at values around 0.1-10 kS/cm for different polymers. The highest values reported up to now are for the conductivity of stretch oriented polyacetylene with confirmed values of about 80 kS/cm. (See, e.g., Cattena, Carlos J., et al., Physical Review B82 (14): 144201 (2010); Heeger, A. J., et al., Reviews of Modern Physics 60: 781 (1988); Heeger, A. J., Handbook of Organic Conductive Molecules and Polymers; Vol. 1-4, edited by H. S, Nalwa (John Wiley & Sons Ltd., Chichester, 1997); Handbook of Conducting Polymers; Vol. 1,2, edited by T. A. Skotheim, R. L. Elsenbaumer, and J. R. Reynolds (Marcel. Dekker, Inc., New York, 1998); Semiconducting Polymers; Vol., edited by G. Hadziioannou and P. F. v. Hutten (Wiley-VCH, Weinheim, 2007); and Burroughes, J. H., et al., Nature 347: 539 (1990); and Sariciftci, N. S., et al., Science 258 (5087): 1474 (1992), the disclosures of which are incorporated herein by reference.)
Conducting polymers show various promising applications, such as in transistors, sensors, memories, actuators/artificial muscles, supercapacitors, and lithium ionic batteries. (See, e.g., Aleshin, A. N., Adv. Mater., 2006, 18, 17-27; Virji, S. et al., Nano Lett., 2004, 4, 491-496; Huang, J. X. et al., J. Am. Chem. Soc., 2003, 125, 314-315; Alici, G. et al., IEEE-ASME Trans. Mechatron., 2008, 13, 187-196; Hui, P. et al., Biomaterials, 2009, 30, 2132-2148; Tseng, R. J. et al., Nano Lett., 2005, 5, 1077-1080; Baker, C. O. et al., Adv. Mater., 2008, 20, 155-158; Spinks, G. M. et al., Adv. Mater., 2006, 18, 637-640; Wu, Y. et al., Synth. Met., 2006, 156, 1017-1022; Wang, Y. G. et al., Adv. Mater., 2006, 18, 2619-2623; and Oyama, N. et al., Nature, 1995, 373, 598-600, the disclosures of each of which are incorporated herein by reference.) Despite the promise presented by these materials, a number of challenges exist to their broad adoption and use in electrochemical devices. One challenge is usually the low solubility of these polymers. In addition, conductive polymers enjoy few large-scale applications due to their poor processability, the manufacturing costs associated with the material, material inconsistencies, toxicity, poor solubility in solvents, and inability to directly melt process. In fact, the poor processability for many polymers requires the introduction of solubilizing substituents, which can further complicate their synthesis. (See, e.g., Hush, Noel. S., Annals of the New York Academy of Sciences, 1006:1 (2003); B A Bolto, et al., Australian Journal of Chemistry, 16(6) 1090, (1963); De Surville, R. et al., Electrochimica Acta, 13: 1451 (1968); Diaz, A; Logan, J., Journal of Electroanalytical Chemistry, 111: 111 (1980); Blois, M. et al., Biophysical Journal, 4: 471 (1964); Nicolaus, R. et al., Tetrahedron, 20 (5): 1163 (1964); and Nicolaus, R. A. and Parisi, G., Atti Accademia Pontaniana XLIX, 197-233 (2000); and McGinness, J. et al., Science, 183 (127): 853-5 (1974), the disclosures of each of which are incorporated herein by reference.)
Many of these problems are being addressed through the formation of nanostructures and surfactant stabilized conducting polymer dispersions in water, including nanofibers and PEDOT:PSS. These nanofiber conducting polymers are rapidly gaining attraction in new applications because they are highly processable materials with better electrical and physical properties. (See, e.g., Tran, H D, et al., ACS NANO, 2008, 2(9), 1841-1848; Tran, H D, et al., Adv. Mater., 2009, 21, 1487-1499; and Li, D, et al., Accts. Chem. Res., 2009, 42(1), 135-145, the disclosures of each of which are incorporated herein by reference.) However, despite the promise offered by these conjugate polymer nanofibers, thus far no practical electrochemical devices have been developed using these materials.
Accordingly, a need exists for improved electrodes formed from conductive polymer structures capable of being used for ion storage in electrochemical devices.